1. Field of the Invention
The present invention relates to a fall detection device for detecting whether a device is falling or not on the basis of acceleration, and a magnetic disk drive including the same.
2. Description of the Related Art
Devices for detecting whether a device is falling or not are disclosed in Japanese Unexamined Patent Application Publication No. 2000-241442 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 08-221886 (Patent Document 2), and Japanese Unexamined Patent Application Publication No. 2000-298136 (Patent Document 3).
A fall detection device described in Patent Document 1 detects whether the device is falling based on whether a differential output signal of a detection signal of an acceleration sensor is substantially 0.
A device described in Patent Document 2 obtains a speed signal by integrating an output signal of an acceleration sensor once and, when the speed is greater than or equal to a reference, it is determined that the device is falling.
A device described in Patent Document 3 determines whether or not the device is falling based on an output of an acceleration sensor, a speed signal generated by integrating the output once, and a distance signal generated by integrating the output twice.
The structure shown in Patent Document 1 requires that a detected acceleration be substantially 0. Thus, an acceleration sensor that can detect direct-current acceleration (DC acceleration) is essential. The DC acceleration output must be adjusted to be substantially 0 or less in the case of 0G application (i.e., at the time of a fall). At the same time, the DC acceleration output must also be adjusted to be substantially 0 or less regardless of external factors, such as temperature and humidity, or aging. Therefore, the acceleration sensor and a circuit using the same are complicated and expensive.
The device described in Patent Document 2 obtains the speed of falling by integrating acceleration once (linear integration) and determines whether the device is falling on the basis of whether the speed is greater than or equal to a reference. The device described in Patent Document 3 detects a fall based on DC acceleration, a speed signal generated by integrating a DC acceleration output once, and a distance signal generated by integrating the output twice. In either case, as will be described later, there are many detection errors, such as falsely detecting as a fall a change in the tilt of a mobile apparatus containing the fall detection device.
More specifically, when the tilt of the direction of an acceleration detection axis relative to the direction of acceleration of gravity changes by θ, the output changes by (1−cos θ). For example, even when the acceleration detection axis, which is directed along the direction of acceleration of gravity, becomes tilted by about 90° relative to the direction of acceleration of gravity, a “fall” is detected.
In Patent Document 3, the output of the acceleration sensor when the device is falling must be at least adjusted to be equal to or less than a threshold, and the sensor output must also be set to 0.2 G or less regardless of external factors, such as temperature and humidity, or aging.
These restrictions prevent a reduction of the cost of the device.
The device described in Patent Document 1 determines that the device is falling when a state in which both the differential output of the acceleration detection signal and the acceleration detection output are substantially 0 continues for a predetermined period of time. However, the differential output is output instantaneously for the acceleration of gravity induced on the acceleration sensor at the start of a fall, and thereafter, the differential output converges to 0 in accordance with a time constant of a differentiator. Therefore, the fall determination is delayed by the amount of the time constant.
In the case that DC acceleration must be detected, as in Patent Document 1 and Patent Document 3, the direction of acceleration applied to the acceleration sensor must be detected to determine whether the acceleration is DC acceleration or not. In an environment in which it is impossible to detect the falling direction of a device to be detected, accelerations in the directions of three axes that are perpendicular to one another must be detected. A magnetic disk drive, such as a hard disk drive, is sensitive to acceleration in a vertical direction relative to a recording surface of a magnetic disk, and thus, it is only necessary to detect a fall in that direction. However, the above-described method of detecting DC acceleration theoretically requires three acceleration sensors and a circuit for processing sensor outputs, resulting in an expensive system.